The present invention relates to cushions, and more particularly to cushions utilizing air and gel to dampen vibration, absorb shock and distribute pressure.
Those who ride bicycles and other types of transport often suffer painful tissue damage on their hands or buttocks from contact with hard surfaces. To avoid this damage cushions of various shapes and configurations are employed to deal with the complex effects of prolonged contact, vibration, and shock at the human body/vehicle surface interface.
Prolonged contact of tissues with a hard surface can restrict the flow of blood. This may lead to ischemia of the skin, sometimes referred to as "saddle sores", and, ultimately, more serious ischemic ulcers. The severity of the tissue damage depends on both time and intensity of pressure at the point or area of contact. Elderly or crippled persons are often tormented by the acute pain of prolonged contact when they sit for long periods of time in a wheelchair without repositioning. Paradoxically, active sports enthusiasts may also suffer from the effects of prolonged contact; bicycle riders in long races have experienced saddle sores. The distressing effects of prolonged surface contact may be reduced through use of a cushion which increases the area of contact by conforming to the irregular surfaces of the body. This reduces pressure on the tissues and permits circulation.
Vibration is a regular periodic transfer of energy which may be transmitted to a rider by contact with a surface having rapid rhythmic movement back and forth. Among other effects, vibration causes fatigue. Bicycle riders, for example, may suffer fatigue from vibration inflicted by a bicycle driven at higher speeds over a hard surface, such as a paved street. These effects may be reduced by interposing an energy absorbing medium between the vibrating surface and the body. This lowers the amplitude of the vibration on the body, which, consequently reduces the amount of energy that is transferred.
Shock, the transfer of high amplitude energy to a person, can cause serious trauma--tissues or bones may be fractured or crushed instantly upon impact with a hard surface. Shock is likely to occur, for example, when one rides a bicycle over a rough surface, such as an unpaved road. When the bicycle hits an obstruction, such as a rock or pothole, the abrupt displacement of the vehicle is transmitted directly through the rigid frame, seat, and control surfaces to the rider. Shock may be reduced by interposing a medium capable of absorbing high displacement amplitude energy between contact surfaces and the rider.
Vehicle riders are often exposed to all of these forces simultaneously. Modern vehicles are designed for flexible multi-mode operation over a variety of surfaces. For example, self propelled wheelchairs are capable of traversing stairs as well as smooth surfaces. High tech bicycles are designed for both on and off the road operation. The use of vehicles over various surfaces may subject the rider to the simultaneous effects of shock, vibration, and long term contact pressure. Air has been and is used as a cushioning medium in pneumatic cushions. Air absorbs shock because it acts like a spring when constrained in a container of variable volume; the container becomes progressively stiffer when compressed by shock displacement. Pneumatic cushions thus have a positive vertical spring rate, which determines how far a person sinks into the cushion.
Pneumatic cushions when maintained at a low pressure have proven unsatisfactory in that they may bottom out against a hard underlying surface when compressed by a shock displacement equal to the thickness of the cushion, which may expose the user to vibration and excessive contact pressure. Conversely, when at high pressure the pneumatic cushion may provide satisfactory shock absorption and distribution of pressure, but still permit the transmission of vibration energy.
Gel-like viscoelastic materials have also been used as a cushioning medium. Such materials are generally incompressible and exhibit both viscous and elastic properties. When subjected to vibration, viscoelastic materials produce a retardation of the effect of the forces acting on the body, which introduces hysteresis into the cycle, and represents a loss of resilient energy. This reduces or dampens vibration. Moreover, upon contact with an object, viscoelastic materials deform slightly thereby absorbing shock and conforming to the surface of the object. This action produces an essentially hydrostatic pressure envelope around the object, which is desirable because it maximizes the area of contact and lowers pressure in tissues.
Gel-like viscoelastic materials also have proven difficult to use because they are heavy, and do not absorb shock adequately due to their relative incompressibility.
As used herein, the term "viscoelastic material" means a substance which when subjected to a very rapidly applied stress, undergoes a deformation proportional to the stress and shows a recovery if the stress is very rapidly removed. If the stress is applied slowly or over a long period of time, the substance behaves somewhat like a viscous liquid: it will show a continued deformation with time, the rate of deformation being proportional to the applied stress. Examples of substances meeting this requirement include silicone gels, vinyl plastisols and polyurethane elastomers.